The present invention relates to expansion valves of the type employed for controlling flow of refrigerant fluid in air conditioning and refrigeration systems. Typically, in air conditioning systems, such as those employed for automobile passenger compartment cooling an expansion valve throttles the flow of pressurized liquid refrigerant at relatively high pressures from the condenser to provide relatively low pressure flow to an evaporator and return therefrom to the compressor. In particular, expansion valves employed for controlling flow of liquid refrigerant to an evaporator in an automotive air conditioning system are of the type known as a "block" valve, wherein the valve body or block has a separate return flow passage therethrough in which vaporized refrigerant discharged from the evaporator passes to permit temperature and pressure thermal sensing thereof for control purposes.
It is known, for example to provide an actuator rod for moving the expansion valve and to expose the rod to the refrigerant flowing in the return passage to the compressor for heat transfer therebetween. It is also known to employ the heat transfer through the rod to provide a temperature signal which in turn operates a pressure-responsive means connected to the actuator rod for controlling the function of the expansion valve in response to changes in the temperature of the refrigerant discharging from the evaporator. It is also known to provide a fluid filled chamber having pressurized fluid therein which acts upon a diaphragm as the pressure-responsive means to move the valve actuator rod control member; and, to having a portion of the rod filled with the pressurized fluid to thereby be in heat transfer relationship with the refrigerant flowing through the return passage to the compressor inlet.
It is known to provide a refrigerant expansion valve which has a hollow tubular member 1 attached to a diaphragm 2 sensing the pressure in the fluid filled chamber formed by a capsule 3 above the diaphragm with the hollow actuator rod extending through the compressor return passage and adapted for moving the control valve member. In valve constructions of this latter type, the hollow actuator rod may experience sudden changes in the refrigerant temperature being sensed, which results in a prompt change in the pressure in the fluid filled chamber which acts upon the diaphragm. Sudden changes in pressure in the fluid filled chamber create a corresponding change in the flow through the control valve, which can result in overcontrol or undesirable oscillations in refrigerant flow in the evaporator. These transients can result from engine speed changes, brief changes in condenser or evaporator fan speeds accumulated oil cascading in the evaporator or other causes.
The time constant for this sensed temperature change, and resulting pressure change, is typically on the order of two seconds to achieve 63% of the eventual change or asymptotic limit. However in some systems, it has been found necessary to provide a longer time constant to prevent the system from responding to such transients. In systems requiring an extended-time response period, constants on the order of five seconds minimum and approximately 40 seconds maximum have been needed.
In order to dampen or retard the effects of temperature transients in refrigerant discharging from the evaporator, it has been the practice in known valves to insulate the actuator rod with a jacket 4. This technique has not been entirely satisfactory for ensuring a desired action of the controls system; and, difficulties have been encountered in providing the desired rate of response where time constants longer than ten seconds are needed with such insulation in a design which permits mass production of valves for passenger automobile air conditioning systems. It has thus been desired to provide a low cost, easy-to-manufacture thermostatic refrigerant expansion valve which has an easily alterable speed of thermal response for achieving the desired action for controlling flow in a refrigeration system such as an automotive air conditioning system.